A prior art digital induction motor control system is shown in U.S. Pat. No. 4,361,792 issued Nov. 30, 1982 and assigned to the assignee of the instant application. The system disclosed in that prior art patent derives a calibrated phase angle and then controls the operation of the motor based on that phase angle, irrespective of variations in motor loading. In this way energy consumption is reduced. Referring to FIG. 1 of U.S. Pat. No. 4,361,792, after the motor has gotten up to speed the count in incremental counter 38 and delay counter 36 is zero. The number in phase angle counter 34 corresponds to the phase angle between motor voltage and motor current zero crossings. The count in the incremental counter is incremented every other cycle, which causes the count in the delay counter to be similarly increased. This increased delay in firing the triac reduces the phase angle and, therefore, the count in the phase angle counter. Eventually, the number in the phase angle counter will be approximately equal to the number in the delay counter. The number in the phase angle counter can then be stored in the phase angle register as the calibrated phase angle and used to operate the system.
Deriving the calibrated phase angle in this manner, however, means that the number stored in the phase angle register as the calibrated phase angle depends, in part, on the initial motor loading. Greater power savings can be obtained if the motor happens to be heavily loaded when the calibrated phase angle was derived. The prior art also discloses how to modify the initial calibrated phase angle so as to yield a refined calibrated phase angle. See column 5, line 3 through column 6, line 3 of U.S. Pat. No. 4,361,792.